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22 Mar 2011
Changes in chromosome number leading to alterations in phenotype
Gene mutations are sources of change, diff genotypes
Genome can be remodelled by large scale chromosomal changes
oMany genes affected simultaneously
oDetected by molecular and genetic analysis, microscopy
EX. Downs syndrome
Plant breeders often manipulate chromosome number to improve or enhance crops
Euploidy organisms or cells that have complete set of chromosomes
Diploid individuals humans have 2 sets of chromosomes normally
If an individual lacks entire set of chromosomes
If one entire set is missing, monoploidy results
Polyploidy occurs in an individual that has more than normal number set of
oTriploid 3 sets
oTetraploid 4 sets
oNotation is N
oThese are all euploidy
Aneuploidy refers to changes in part of chromosome set, not entire set
In other words, loss or gain of one or more chromosomes
Monosomy when one specific chromosome is missing
Trisomy causes Downs syndrome
oGain one extra chromosome
Tetrasomy gain two extra specific chromosomes
Differs from wildtype by PART OF CHROMOSOME SET
Monoploids are different from haploids
Normally haploid has one chromosome set wildtype is haploid in this case
Monoploid arises due to aberrant euploidy
Males of bees, wasps, etc. are monoploid
oDevelop by parthenogenesis
oSingle set of chromosomes bypass meiosis, producing gametes by mitosis
Usually lethal in other systems and species
oUnmask recessive lethals
oGenetic lode individuals will have wildtype copy that will mask the
recessive lethal allele
Monoploidy cases recessive lethal are now unmasked
So mutant phenotypes expressed
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Hence monoploids in other species tend to be inviable
If survive to adulthood, no meiosis, and individuals are sterile
because germ cells have no chromosomes to pair with
Created experimentally
oTake pollen grains (male germ cells) cold treated and plated onto agar
and grown on this agar surface
oGrowth of haploid embryoids, treated with plant hormones to induce them
to grow into monoploid plant that is usually sterile
oTo visualize recessive traits do not need to cross to show homozygous
recessive conditions
oAnd also useful by being able to introduce mutations in monoploid cells
and converted into diploid plants that are fertile
Alter monoploid into fertile diploid plants
Very common in plants
Associated with origin of new species
Normally correlates with size and vigour
Typically proportional in size size of berry to stem
Higher ploid level, typically larger the size
Two types
1. Autopolyploids
oWhen multiple chromosome sets originate within one species
oEx. autotriploids = normally autopolyploids
Produced from fertilization of diploid and monoploid gametes
Can arise spontaneously or by manipulation
Sterile because of formation of aneuploid gametes
Ex. seedless watermelons
Due to presence of unpaired chromosomes during meiosis
One homolog is left unpaired
oEither go to one pole or other pole randomly
oCells with two of one chromosome type; and other
with one of other chromosome type
oSegregation pattern happens for every chromosome
type: 1, 2, 3,.
oNormally, will have unbalanced gametes
One of every chromosome type in every cell
for balanced gametes
oUnbalanced gametes and anuploid results in
Autotetraploids arise due to doubling of any two chromosomes to become a 4n chromosome
Spontaneously occurs or induced by drug
Colchicines prevents polymerizaiton of microtubules
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